1. Field of the Invention
The present invention relates to a zoom lens and in particular, although not exclusively, to a subminiature zoom lens that has an increased angle of view and high performance used in a small image pickup apparatus.
2. Description of the Related Art
In general optical systems, the ideal image height Y is represented by Y=f tanθ where f is the focal length and θ is the angle of view.
In such an f tanθ projection zoom lens, the amount of light at edges of a screen (peripheral brightness) decreases with an increase in the angle of view in accordance with the cosine fourth law. Therefore, for a wide-angle zoom lens whose angle of view exceeds 80°, the peripheral brightness largely decreases.
In digital image pickup equipment, luminance information on a captured image can be electrically modified. However, because luminance information on the edges is lost, a circuit for correcting the luminance at the edges is necessary.
For an image pickup system of, for example, a digital camera, it is, therefore, difficult to simplify the entire apparatus. In a general wide-angle zoom lens, distortion at the wide-angle end is corrected, and aperture efficiency is increased in order to avoid light falloff at the edges. As a result, an effective diameter of a lens element closest to an object is large.
A zoom lens has been discussed whose lens construction is simplified by actively causing negative distortion and whose angle of view reaches a range covered by a fish-eye lens at the wide-angle end. U.S. Pat. No. 5,724,193 discloses one such zoom lens that is constructed of two or three lens units including leading lens units with negative and positive refractive powers, respectively. In this zoom lens, a distortion of −50% or more is caused at the wide-angle end, thus enabling the wide-angle zoom lens to include a first lens unit of simple lens construction in which lens elements are all spherical, to consist of 7 to 12 lens elements in the entire system, and to achieve an angle of view exceeding 120° at the wide-angle end.
The number of lens elements in the zoom lens disclosed in U.S. Pat. No. 5,724,193 is relatively large at 7 to 12. Additionally, because a lens element closest to the object in the first lens unit is a negative meniscus lens, the effective diameter increases with an increase in the angle of view. Therefore, miniaturization is inevitably difficult.
An existing endoscope objective lens intended for close observation is a zoom lens of a fish-eye lens type having an angle of view of more than 80° at the wide-angle end.
Japanese Patent Laid-Open No. 2000-330024 and U.S. Pat. No. 6,433,937 disclose a zoom lens for a magnifying endoscope that includes three lens units having negative, positive, and negative refractive powers. In both patent documents, causing a distortion of approximately −50% at the wide-angle end enables an endoscope objective lens to have an angle of view of 80° or above with five to seven lens elements.
However, in both patent documents, because a first lens unit includes a negative lens element whose first surface is plane at a position closest to an object, the effective diameter increases with an increase in the angle of view. Therefore, miniaturization is inevitably difficult.
Recently, for miniaturization of the entire apparatus, a zoom lens used for a cellular phone or a small digital camera has been required to have a smaller number of lens elements and a shorter overall lens length. Additionally, the zoom lens has been required to have a wide angle of view for shooting, in particular, that exceeds 80° and a smaller falloff in the peripheral brightness.
In general, an increased number of lens elements in each lens unit in a zoom lens increases the length of each lens unit along the optical axis, and an increased amount of travel of each lens unit during zooming increases the overall lens length. As a result, miniaturization of the overall lens system is difficult.
To address the problems, a smaller number of overall lens elements and a smaller amount of travel of each lens unit during zooming would be useful. Additionally, since, in a wide-angle zoom lens, the peripheral brightness decreases with an increase in the angle of view for shooting, a smaller light falloff at edges of a screen is also greatly useful.